Changing the chapter numbering is easier than
eliminating it for the first chapter. That's because often the word
"Chapter" is added to the chapter number, and all those instances need to be
handled if you eliminate it for one chapter.

Chapter numbers are generated by processing a
chapter in mode="label.markup" (true of most element numbering as well).
Something like this would work to start on zero:

It just counts the number of preceding
chapters. The first one will have a count of zero.

The original template in this mode is in
common/labels.xsl, and you will see that it is a bit more complex because it
handles many options that come from parameter settings. If you don't need
to accomodate those variations, then this simple template would work.
Otherwise customize the original.